U.S. Pat. No. 3,296,217, U.S. Pat. No. 3,900,450 and British Patent Specification No. 854,223 disclose various processes for the preparation of a fiber-forming polycarbonamide from an aqueous solution of a salt derived from substantially chemically equivalent proportions of an aliphatic diamine and a dicarboxylic acid. In these processes aqueous vapors evolving from a heated reaction zone into which the salt is fed are fractionated in a rectifying zone so that unreacted diamine contained in those vapors is removed therefrom and returned to the reaction zone along with liquid reflux water.
The effectiveness of such rectifying zones in recovering unreacted diamine depends, among other things, upon the operating reflux ratio, i.e., the ratio by weight of liquid reflux water to water vapor flowing through the zone. Generally as the reflux ratio is raised, more liquid water is returned to the reaction zone and more diamine is recovered, thus reducing ingredient costs. Such recovery also reduces environmental pollution problems occasioned by release of diamine to the atmosphere. Unfortunately, higher reflux ratios require more energy to operate than lower ones, since the liquid reflux water being returned to the reaction zone must be re-boiled. Consequently, recent increases in energy costs have made some recovery processes, which previously were economical to operate, no longer economical to do so. In other words, the cost of process energy can no longer be offset by savings resulting from recovered ingredients. Thus, more expensive energy has created an incentive to reduce the reflux ratio in some recovery processes as described above at a potential sacrifice in environmental quality where the value of the diamine recovered no longer offsets the cost of the process energy required to recover it.